<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .father {
      /* 利用flex实现 */
      display: flex;
      /* 主轴水平居中 */
      justify-content: center;
      /* 侧轴水平居中 */
      align-items: center;
      width: 300px;
      height: 300px;
      background-color: pink;
    }

    .son {
      width: 200px;
      height: 200px;
      background-color: purple;
    }
  </style>
</head>

<body>
  <div class="father">
    <div class="son"></div>
  </div>
</body>

</html>